1. Field of the Invention
The present invention relates to an electromagnetic assembly for mosaic printing head and a related manufacturing method.
2. Description of the Prior Art
It is known that the mosaic serial printers have been widely used as console printers in data processing systems as well as computer printing terminals. Such printers are required to have high reliability, low cost and high performances (speed, printing quality). A considerable portion of the cost of these printers is determined by the cost of the printing heads which require precise operations of assembling and setting. Reliability, printing speed and printing quality of such printers essentially depends on the printing heads too. Therefore, considerable attention of serial printer manufacturers is devoted to the study, the development and the improvement of the print heads. This is confirmed by the large number of patents and patent applications relating to the design and manufacturing details of such print heads.
According to the most recent embodiments, the print heads include a plurality of electromagnets. A movable armature coupled to each electromagnet controls the axial movement of a needle, so that the needle causes an impression on a printing support via an inked ribbon. A typical example of such printing heads is given by U.S. Pat. No. 3,889,793.
In order to facilitate the assembling and setting operations, the design of modern printing heads has evolved so that they may be formed by functional assemblies which may be easily joined. An assembly usually comprises the needles and the related guides and support elements (needle guide assembly). Another assembly comprises the electromagnets and their related movable armatures with their related clamping and positioning means and their related devices for adjusting the movable armature stroke (electromagnetic actuator assembly). An example of printing heads arranged according to such criteria is given by U.S. Pat. No. 4,260,270.
In these printing heads, the electromagnetic actuator assembly includes a ring-shaped support element on which a plurality of magnetic cores, generally constituted by a yoke and two columns, is fixed. A winding is arranged around a column of each core and provides the magnetization of the magnetic circuit constituted by the core and by a movable armature closing the magnetic circuit. The several armatures, each one coupled to a core, are properly positioned by a suitable retaining element. The electromagnet support element and the cores with their respective windings form a unitary set which we shall call the electromagnet assembly. Such electromagnet assembly, together with the movable armatures and the respective retaining and positioning devices, forms the electromagnetic actuator assembly. As is known, the printing head is mounted on a movable carriage of the printer. Therefore, the energization of the several windings is obtained by connecting their terminals to a feeding electronic circuit placed inside the printer through a flexible cable having several conducting leads. Connection to the cable can be direct or through connectors. The connection of the several windings to the flexible cable or to the connector is expensive because it requires a manual connection operation. Moreover, it is delicate and is responsible for frequent failures which are due to the printing head and carriage vibrations and reduce the equipment reliability. The mounting of the magnetic cores is critical too, although obtained by shrinking of the magnetic core sheets into suitable slots or by calking, they are responsible for possible breakage. Attempts have been made to obviate these inconveniences by partially encasing the windings and the cores in thermoplastic resins poured on them. This sticks the electromagnet support elements, the magnetic cores and the windings together. The above arrangement is, for example, described in U.S. Pat. No. 4,049,107. But such arrangement avoids only in part the cited inconvenience because the connection of the windings remains a critical element and adds an operation and therefore a cost to the manufacturing process of the printing head. The present invention overcomes such disadvantages.